Observing the night sky can be quite easy under the right conditions. In its simplest form, all that is required is that one be outside after dusk on a cloudless evening, and just look up. A moonless sky away from the city lights will reveal a sky full of stars while a city observer under a full Moon will see just a few stars. This simplest form of observing the night sky is all that some stargazers will ever want to know how to do, while other stargazers will want to make some sense of the sky, and perhaps even be able to find or identify constellations. Knowing where to start can be intimidating for some, so my sincere hope is that this article enables an enhanced observation experience by gaining an understanding of how a star chart is used to find and identify constellations and other celestial objects.
The Sky – A Stargazer’s Working Definition
A star chart will be of little use until an observer understands the sky that the chart represents, so developing a basic understanding of the sky is important. Defining the sky is rather simple: walk out into the middle of a large, open field, or just imagine yourself standing there. Lift one arm until it is horizontal, and point in any direction at the “line” where the sky meets the earth. This is a point (see what I just did there?) on the horizon. Keeping your arm extended horizontally, turn about in a complete circle, and the circle traced out by the extended finger has traced out all of the points that make up the horizon, which we will simply refer to as the horizon. All that is below the horizon is earth, and all above is sky.
Next one must determine which direction on the horizon is north to permit proper alignment of the star chart. Please read Cardinal Directions for Casual Observers for some “pointers” on this topic.
Zenith is another point in the sky that is an aid in locating constellations. To locate this point, imagine a line drawn from the center of the Earth, up to its surface at point at which you are standing and through your body, and extending out of the top of your head. The point at which this line touches the celestial sphere above you is the zenith. No matter where on the surface one might be standing, or what time or what day it is, this simple rule defines the zenith. Zenith is located at the center (not coincidentally) of the sky chart and is indicated by the circled “Z.”
All of the stars and other objects that might be seen in the sky are at varying distances from us. Since even the nearest of these objects are so far away, our mind perceives them as all being at the same indeterminable distance away. For this reason and for the purpose of this amateur stargazers, the sky and all that it holds can be thought of as the celestial sphere – much like the inside of a planetarium dome. Stars are fixed points upon this sphere that all move together across the sky. Imagine the Earth at the center of this sphere, and that the sphere surrounds the entire Earth.
In reality, the Earth spins on its axis beneath the celestial sphere. The point on the Earth’s surface from which we happen to be observing circles the Earth’s once each day, and we observe the celestial objects passing through our field of view, or the sky, from east to west. Our perception is that the Earth is fixed, and the celestial sphere turns above us. For casual observational purposes, and for ease of explanation, the remainder of this discussion will assume the perceived motions.
The Star Chart
At the beginner level, a chart prepared for the observers location and time is required. Below is a representative chart of the chart that I post in monthly editions of Scope Out Next Month. This particular chart represents the celestial sphere over Ashton, Maryland at nightfall on March 15, 2015. For casual observational purposes, this chart is good at nightfall for the entire month of March 2015, and for mid-latitude northern locations. Be sure to grab an up-to-date chart from the current month’s Scope Out. Click to expand the image if you would like, and take a few moments to look it over before reading on.
The outside edge of the chart is a circle, which represents the circular horizon that we traced out early on. Note the N, S, E, and W points at the edge of the horizon. Notice that the east and west points are on the sky chart are opposite of the east and west points that we are accustomed to seeing on a map. Why? Because instead of looking down on a map, we are now looking up at the sky!
Observing the Night Sky with a Sky Chart
First, grab an up-to-date sky chart from the Scope Out archive, get comfortable and get aligned. Comfort for star gazing would be lying flat out on the ground, perhaps on a padded surface, or reclined in suitable lawn furniture. Dressing for the current weather conditions is important as well. Being aligned means that the top of one’s head is pointing north. From a reclined position, east will be to the observers left, west to the right, and south toward the feet. Now hold the chart up against the sky, and identify the constellation that you wish to find. Using the example sky chart at Ashton, Maryland at nightfall on March 15th, 2015 as an example, one might locate the zenith, and look south and slightly west to find Orion. Looking further south, toward the feet, I could find Canis Major. I could find Leo by looking to my right, halfway down toward the horizon, and I could look up toward the top of my head to find the northern constellations, like Ursa Minor, or the Little Dipper.
That’s all there is to observing the night sky with a sky chart prepared for the date, time, and location of the observer. Understanding a Star Chart, which builds upon the concepts presented here, is the next logical progression to constructing a more complete understanding of the night sky.
How to begin Observing the Night Sky, in my mind, is probably the most important article that I have posted to my AstroLog. I would appreciate your feedback to make easier to follow or understand, and your questions are always welcome. Drop me a line at firstname.lastname@example.org.
– Horizon and Coordinate System, http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec2.html. Accessed by Jim Johnson on January 19, 2015.
– Celestial Sphere, ASTR 1230 (Majweski) Lecutre Notes. http://www.astro.virginia.edu/class/majewski/astr1230/LECTURES/LECTURE2/lecture2B-s13.html. Accessed by Jim Johnson on January 19, 2015.- — – Stargazing Couple, http://www.lovethispic.com/image/3649/star-gazing-couple. Accessed by Jim Johnson on January 19, 2015.
© James R. Johnson, 2015.